I have heard this from multiple sources, but I still don't quite get what it does/ what it is?
Can you give the Denzel Washington from Philadelphia-style explanation (like we're 8-year-olds)?

At it's absolute simplest level...

Plex is a media server that runs on your computer and makes your media accessible through a number of Plex apps.

Again, at most basic level, you install the client-side server on your PC and import your videos, music, etc. Then you can use a Plex app (ie. on a Roku or phone or tablet) to access and play the media stored on your PC's Plex server.

It basically just makes media content management extremely simple because you only ever have to manage it in one spot (your PC) but can then access it from a multitude of platforms. There's more to it in terms of presentation style, metadata, etc. that people like a lot, but at its most basic level it's an easy way to organize and access shitloads of media without having to copy it to multiple devices.